This study attempts to produce an experimental animal model of chronic diffuse interstitial pulmonary fibrosis in man. Guinea pigs immunized with complete Freund's adjuvant were repeatedly exposed to PPD in aerosol during an 8 week period. Results derived from lung volume measurements obtained at sacrifice and from histologic studies are in general agreement. The lungs of animals treated as described above and sacrificed 48 hours after PPD inhalation show obvious parenchymal lesions and a reduced lung volume. Study of controls demonstrates that these changes differ from those related to Freund's adjuvant and cannot be attributed to spontaneous lung disease. However, significant lesions affecting lung volume did not persist 3 months after the last PPD aerosol exposure. Current and projected efforts are designed to enhance the severity of the lung lesions and to produce permanent respiratory impairment.